1. Field of the Invention
The present invention relates generally to the field of molecular biology. More particularly, it concerns the inhibition and/or inactivation of nucleases (both deoxyribonucleases and ribonucleases) which can degrade DNA (deoxyribonucleic acid) and/or RNA (ribonucleic acid). Inhibition and/or inactivation of nucleases in the present invention employs at least one, and in many cases at least two, nuclease inhibitors. These nuclease inhibitors include anti-nuclease antibodies and non-antibody nuclease inhibitors.
2. Description of Related Art
The quality of an RNA preparation greatly affects the results obtained when analyzing it by a number of different molecular biology techniques such as northern blotting, ribonuclease protection assays and RT-PCR (Reverse Transcriptase-Polymerase Chain Reaction). Degraded RNA will produce a lower signal than in an equivalent intact RNA sample.
RNA is much more susceptible to degradation than DNA (Sambrook et al., 2001). RNA is readily hydrolyzed when exposed to conditions of high pH, metal cations, high temperatures and contaminating ribonucleases. A major cause of RNA degradation is ribonuclease contamination, and this must be guarded against in virtually all RNA-related procedures, including RNA isolation, mRNA purification, RNA amplification, RNA storage, northern blotting, nuclease protection assays, RT-PCR, in vitro transcription and/or translation and RNA diagnostics. In addition to the endogenous ribonucleases from cells and tissues, skin secretions and airborne bacteria and/or fungi are common sources of ribonuclease. To minimize ribonuclease contamination, appropriate precautions must be followed when handling RNA (Blumberg, 1987; Wu, 1997).
Ribonucleases are difficult to inactivate. For example, bovine pancreatic ribonuclease A (RNase A) has no activity at 90° C. However, if the enzyme is quickly cooled to 25° C., the activity is fully restored. This process is known as reversible thermal denaturation. If the RNase A is incubated at 90° C. over time, then decreasing fractions of the activity are recovered at 25° C. This process is known as irreversible thermoinactivation. At 90° C., it takes several hours to inactivate RNase A (Zale and Klibanov, 1986). Much of the lost activity is attributed to disulfide interchange (Zale and Klibanov, 1986). Further, the inventors and others have found that ribonucleases can even withstand autoclaving (121° C., 15 psi, 15 minutes) to some degree. Spackman et al. (1960) tested the stability of RNase A and concluded that it was stable to heat, extremes of pH, and the protein denaturant, urea, results emphasizing the difficulty researchers have had inactivating ribonucleases. For the above reason, a variety of methods other than heating have been developed to inhibit or inactivate ribonucleases. These methods, and their disadvantages, are described below.
In one method of destroying RNases, diethyl pyrocarbonate (DEPC) is added to final concentration of 0.1% to molecular biology reagents, glassware or electrophoresis apparatus, followed by incubating at 37° C. for several hours and then autoclaving for 15-20 minutes to destroy the DEPC (Wolf et al., 1970). DEPC reacts with the ε-amino groups of lysine and the carboxylic groups of aspartate and glutamate both intra- and intermolecularly (Wolf et al., 1970). This chemical reaction forms polymers of the ribonuclease. However, there are several disadvantages to using DEPC: (1) It is a possible carcinogen and is hazardous to humans; (2) some commonly used molecular biology reagents such as Tris react with and inactivate DEPC; (3) treatment of samples with DEPC is time-consuming; (4) DEPC reacts with the adenine residues of RNA, rendering it inactive in in vitro translation reactions (Blumberg, 1987) and 5). If all of the DEPC is not destroyed by autoclaving, remaining trace amounts may inhibit subsequent enzymatic reactions.
Traditionally, RNA is stored in DEPC-treated water or TE buffer. However, the RNA is not protected from degradation if the sample or the storage solution has a minor ribonuclease contamination. It has been suggested that RNA be stored in ethanol, formamide, or guanidinium to protect an RNA sample from degradation because these environments minimize ribonuclease activity (Chomczynski, 1992; Gilleland and Hockett, 1992). The obvious disadvantage is that the RNA sample cannot be directly utilized for analysis or enzymatic reactions unless the ethanol, formamide, or guanidinium is removed.
Guanidinium thiocyanate is commonly used to inhibit RNases during RNA isolation (Chomczynski and Sacchi, 1987; Sambrook et al., 2001). A high concentration of guanidinium thiocyanate combined with β-mercaptoethanol is used to isolate RNA from tissues, even those that are rich in ribonucleases, such as pancreas (Chirgwin et al., 1979). Guanidinium is an effective inhibitor of most enzymes due to its chaotropic nature. However, if RNA is dissolved in guanidinium, then it must first be purified from the guanidinium prior to being used in an enzymatic reaction.
Vanadyl-ribonucleoside complexes (VRC) may be used to inhibit RNases during RNA preparation (Berger and Birkenmeier, 1979). The drawback to using VRC, is that VRC strongly inhibits the translation of mRNA in cell-free systems and must be removed from RNA samples by phenol extraction (Sambrook et al., 2001).
Favaloro et al. (1980) employed macaloid, a clay, to absorb RNases. A limitation of this method is that it is difficult to completely remove the clay from RNA samples. Other reagents have been used to inhibit ribonucleases including sodium dodecylsulfate (SDS), ethylenediamine tetraacetic acid (EDTA), proteinase K, heparin, hydroxylamine-oxygen-cupric ion, bentonite and ammonium sulfate (Allewell and Sama, 1974; Jocoli and Ronald, 1973; Lin, 1972; Jones, 1976; Mendelsohn and Young, 1978). None of these reagents are strong inhibitors alone. Like many of the RNase inhibitors already described, although these chemicals inhibit RNase activity, they also may inhibit other enzymes such as reverse transcriptase and DNase I. Therefore, the RNA must be purified away from the inhibitory reagent(s) before it can be subjected to other enzymatic processes.
Two types of proteinaceous RNase inhibitors are commercially available: human placental ribonuclease inhibitor (Blackburn et al., 1977) and PRIME Inhibitor™ (Murphy et al., 1995). RNases of the class A family bind tightly to these protein inhibitors and form noncovalent complexes that are enzymatically inactive. The major disadvantage of these inhibitors is that they have a narrow spectrum of specificity. They do not inhibit other classes of RNases. Another disadvantage when using placental ribonuclease inhibitor is that it denatures within hours at 37° C., particularly under oxidizing conditions, releasing the bound ribonuclease.
Heat has been used to inactivate RNase A by mediating the breakage of disulfide bonds. Zale and Klibanov (1986) performed inactivation of RNase A at 90° C. and pH 6.0 for 1 hour, which induced the following chemical changes: disulfide interchange, β-elimination of cysteine residues, and deamidation of asparagine. This type of heat treatment did not completely inactivate the ribonuclease. A major disadvantage is that a long-term, high-temperature treatment (90-100° C.) is incompatible with RNA. Such treatment promotes the hydrolysis of RNA. In fact, the inventors have found that total RNA incubated at 65° C. for several hours is almost completely degraded. Thus, treating an RNase sample with extreme heat to inactivate ribonucleases will mediate the destruction of the RNA which the user is trying to protect.